I can't comment due to reputation, but you really need to tell us what version of SQL Server you are running, maybe some more information about how the data is structured and how you're pulling the data into these dashboard. Maybe even how long it's taking and what resources you have available that actually know what they are doing.
That said, it sounds like you have a OLTP database with lots of tables. As I don't know the relationships of these tables or how you are pulling the data from these tables, I can only assume you are pulling data from many of these tables. If optimizing the tables is not helping due to how many joins and records you are pulling, then:
SQL Server Analysis Services
It sounds like you need to create a multidimensional database that can be used for reporting. SQL Server Analysis Services helps you do that by allowing you to define OLAP Cubes in many different structures from MOLAP to ROLAP.
Multidimensional Database (Data Warehousing)
Another way is creating a new database that is going to be the foundation for your multidimensional data. Therefore, you would need to create a complex ETL System within SQL Server that converts those 400 tables into facts and dimensions automatically on a daily basis and pushes it into your new database. This is a similar process to what SSAS is going to do for you when you define cubes in SQL Server.
Preaggregate Tables or Views
If you can't do that yourself, then another way is just building new tables in your database that are just preaggregates of the 400 tables that will be used for reporting. Basically established how you are reading the data for your dashboards and find ways to preaggregate that data into fewer tables before you actually pull it into a report.
Automation & Tools
This (as well building multidimensional data) is accomplished simply by creating stored procedures or SSIS packages and automating the process every day. Then Tableau, SSRS or whatever queries the new tables rather than the previous 400 tables that may be slowing the process.
Hire Someone, You Already Have The Tools
The last and final way is finding a tool that does the ETL for you. There are plenty of ETL vendors out there that can possibly address this problem. But keep in mind, you likely have all the tools you need to do this. You just need to hire the talent to do it either temporarily on contract or full-time.
If I had no knowledge of what I was doing in SQL, I would contract a ETL Developer, SQL Developer or BI Developer to help me. Because why buy another tool box when you already have a good tool box available to you?